Effects of ouabain and EGTA on the inhibitory effect of Ca 2+ on guinea pig stomach smooth muscle (‘Ca reversal’)

Abstract

Guinea pig stomach smooth muscle is known to show sustained contraction when treated with carbachol (CCh) in Ca-free solution containing 2 × 10 −3 M EGTA (‘Ca-free contraction’). After pretreatment of the muscle with a Ca antagonist (D-600, nicardipine or nifedipine), addition of 10 −4 M Ca 2+ (pCa 4.0) relaxed this CCh-induced Ca-free contraction (‘Ca reversal’), but without pretreatment with a Ca antagonist, Ca 2+ evoked contraction. In a Ca buffer containing 2 × 10 −3 M EGTA, low concentrations of Ca 2+, 10 −8 − 3 × 10 −7 M (pCa 8.0–6.5), relaxed the Ca-free contraction dose dependently (‘Ca reversal’), whereas concentrations higher than 10 −6 M Ca 2+ (pCa 6.0 −) reversed this relaxation back to contraction. EGTA at 5 × 10 −3 or 10 −2 M potentiated the Ca reversal, whereas no reversal occurred in a Ca buffer containing 10 −3 M EGTA. This Ca reversal was inhibited in K +-depleted, Ca-free solution and by ouabain in Ca buffer containing EGTA (5 × 10 −3 or 10 −2 M). These results show that Ca reversal of guinea pig stomach smooth muscle occurs at low concentrations of Ca 2+ and that its mechanism is related to ouabain-sensitive processes, such as the function of Na +-K + ATPase.